1. Technical Field
This invention relates to cordless telephones and more particularly to a method and apparatus employed in a cordless telephone for providing increased functionality and for extending the useful battery life of such telephone.
2. Description of the Prior Art
A typical cordless telephone system includes a handset or remote unit and a base unit. The base unit is connected to the telephone lines and includes an antenna, a transmitter and a receiver for communicating a radio frequency carrier signal that is modulated by switching signals and audio signals to and from the handset unit as appropriate. Power for operation of the base unit is obtained from local line power. The handset unit includes a speaker and a microphone, and also an antenna, a transmitter and a receiver for likewise communicating telephone switching signals and audio signals to and from the base unit. Power for operation of the handset unit is obtained from a battery contained therein. This battery is usually charged by the local line power when the handset unit is placed in a cradle located in the base unit.
In the operation of a typical cordless telephone handset unit in originating and receiving a call, the receiver of the handset unit normally remains in an on-condition whenever the handset unit is removed from the base unit. If a user desires to place a call from the handset unit, the transmitter in the handset unit generates a control signal that is transmitted to the base unit. Upon receipt and detection of the control signal, the base unit seizes the telephone lines so as to enable audio signals and switching signals from the handset unit that are received by the base unit within a given audio frequency band to be passed onto the telephone lines. If an incoming telephone ringing signal is sensed by the base unit, reflecting an incoming call, the base unit, in turn, transmits this ringing signal to the handset unit where it is detected by the handset unit receiver which activates a ringer in the handset unit.
High battery drain current limits the period of time for reliable operation of handset units while located remote from associated base units. In order to extend this time and yet insure reliable operation for an extended period, one specific arrangement disclosed in U.S. Pat. No. 4,731,814 and issued to W. R. Becker et al. on Mar. 15, 1988, reduces the battery drain current in the handset unit when the telephone is not in use and the handset unit is on-hook, i.e., no telephone call is in progress. Operation of the handset unit is through a process wherein power to the transmitter in the handset unit is removed and power to the receiver and other selected circuitry in the handset unit is controlled to minimize power consumption when the handset unit is located remote from the base unit and also monitoring for a telephone ring signal from the base unit.
Control of the power to the receiver and the other selected circuitry in the handset unit of the Becker et al. arrangement is achieved by cycling the power to these circuits on and off while the handset unit is in a standby state. And the handset unit only responds to a ringing signal received from the base unit that is accompanied by an identification code that the handset unit recognizes.
While this type of arrangement in a cordless telephone system has been generally satisfactory in the past for those cordless telephones that initiate communications over one of multiple channels and continue to use this same channel for ongoing communications between the handset unit and the base unit, recent rulings promulgated by the Federal Communication Commission in the utilization of spread spectrum systems, including a frequency hopping system, now allow for increased spectral utilization. Frequency hopping systems spread their energy by changing, or hopping the center frequency of the transmission many times a second in accordance with a pseudo randomly generated list of channels. Although ample technical challenges are provided in the design of cordless telephones suitable for operating in these systems, it is nevertheless desirable to provide a cordless telephone that provides the desired functionality, including increased battery life, while operating in a frequency hopping system.